This invention relates in general to a system for chemical mechanical polishing of semiconductor wafers. More particularly, the present invention relates to a linear polisher for the chemical mechanical planarization of semiconductor wafers.
The available systems for the chemical mechanical planarization of semiconductor wafers typically employ a rotating wafer holder for supporting the wafer and a polishing pad which is rotated relative to the wafer surface. The wafer holder presses the wafer surface against the polishing pad during the planarization process and rotates the wafer about a first axis relative to the polishing pad. The polishing pad is carried by a polishing wheel or platen which is rotated about a second axis different from the rotational axis of the wafer holder. A polishing agent or slurry is applied to the polishing pad to polish the wafer. As the wafer holder and the polishing wheel are each rotated about their respective central axes, an arm moves the wafer holder in a direction parallel to the surface of the polishing wheel.
Since the polishing rate applied to the wafer surface is proportional to the relative velocity of the polishing pad, the polishing rate at a selected point on the wafer surface depends upon the distance of the selected point from the axis of rotation. Thus, the polishing rate applied to the edge of the wafer closest to the rotational axis of the polishing pad is less than the polishing rate applied to the opposite edge of the wafer. Rotating the wafer throughout the planarization process averages the polishing rate applied across the wafer surface so that a uniform average polishing rate is applied to the wafer surface. Although the average polishing rate may be uniform, the wafer surface is continuously exposed to a variable polishing rate during the planarization process.
Although the polishing rate is generally proportional to the relative velocity of the polishing pad, other factors as for example fluid dynamic and thermodynamic effects on the chemical reactions occurring during the planarization process influence the actual polishing rate at any given instant in time. These effects are not uniform across the wafer surface during the planarization process. Moreover, instead of "averaging" the effects, the relative rotation of the wafer and the polishing pad contribute to the fluid dynamics and thermodynamics of the reaction.
After a period of time, the polishing pad becomes saturated with deactivated slurry, loose particles, etc. The pad must be frequently roughened to remove such particles from the polishing surface of the pad. For example, a scraping tool is typically mounted in contact with the polishing pad to scrape the loose slurry from the pad surface.
Because of advances in wafer processing technology and semiconductor component structure, uniformly polishing or planarizing a film on the surface of the wafer has become increasingly important. For example, integrated circuits such as microprocessors, controllers and other high performance electronic logic devices have become increasing complex while the size of such devices has decreased substantially. With the multiple wiring layers employed in complex devices, a significant component of the delay in signal propagation is due to the interconnections between the multiple layers. Several multilevel interconnection processes are being developed to reduce the delays associated with interconnect resistance, such as smaller wiring geometry and the use of copper or other materials as interconnect metals. However, the surface of the semiconductor wafer is generally rough. Each wiring layer provides additional circuitry components which project from the wafer surface, producing a rippled effect on the surface of the device. When several layers are formed on the wafer, the uneven topography of the device becomes more exaggerated. Even if the first layer is completely planar, circuitry components of the succeeding layers often produce a rippled effect which must be planarized.
This invention provides a system for uniformly polishing the surface of a semiconductor wafer. The system includes a linear polisher which applies a uniform polishing rate across the wafer surface throughout the planarization process for uniformly polishing the film on the surface of the semiconductor wafer. The polisher is of simplified construction, thereby reducing the size of the machine and making the polisher suitable for even larger-diameter wafers. For example, the linear polisher is approximately 1/5 the size of available machines. The reduced size and simplicity of the machine substantially reduces the manufacturing costs of the polisher. Since less space is required for the polisher, the operation costs are also substantially reduced. Although the overall size may vary, the linear polisher may be only slightly larger than the wafer. The polisher of the invention may have one or more conditioning stations for roughing or conditioning the polishing member during the polishing cycle, ensuring that a uniform polishing rate is applied to the wafer surface throughout the planarization process.